This invention pertains to the art of induction heating devices and, more particularly, to an induction heater wherein the forces on the heater coil cause coil vibration and sound generation.
The invention is particularly applicable to a generally rectangular induction heater for heating workpieces including a coil having a number of coil turns where at least a portion of the turns is supported by a concrete beam to reduce the vibration of the coil and, thereby, reduce sound generation. However, it will be appreciated to those skilled in the art that the invention could be readily adapted for use in other environments as, for example, where similar support members are employed to reduce vibration and sound generation with other types of vibrating items.
It is known that when a current-carrying conductor is in a magnetic field, a force is exerted on the conductor. The direction of the force is at right angles to the conductor and to the direction of the field. The magnitude of the force depends upon the magnitude of the current and upon the strength of the magnetic field. In an induction heater, including a coil in which an alternating current is applied to heat a workpiece, an associated alternating force is induced on the coil which produces vibration and consequent sound generation. For example, in a typical 90.times.24 inch rectangular coil with a conductor current of approximately 4500 amps, the force on the coil is approximately 0.4 lbs. (RMS) per square inch. The sound generated by such a coil force can be in excess of 95 dbA.
The vibration of the coil in an induction heater is a common problem which has several undesirable effects. The vibration will weaken the coil itself since repeated flexing gradually makes the inductor brittle and may ultimately cause cracking. In addition, as noted above, coil vibration generates sound which at a level of 95 dbA may be above the sound regulations for a particular operation, or at least may present an undesirable work environment.
Various forms and types of supports have heretoforce been suggested and employed in the induction heater industry to support a vibrating induction heater coil, all with varying degrees of success. For example, steel beams in combination with laminated support members have been employed (U.S. Pat. No. 3,485,983 to Tama et al.). Although steel beam bracing is functionally efficient, the cost of construction of such a bracing assembly is relatively high and, for economic reasons, other less costly bracing structures are desirable.
Concrete is also known as a relatively cost efficient support casing and refractory material for use in induction heater coils (see U.S. Pat. No. 4,532,398 to Henriksson). However, several problems exist with concrete both in its method of manufacture as an induction heating coil support, and in its ability to withstand the tensile forces generated by the coil during operation. More particularly, concrete typically shrinks when cured and if an induction heater coil is cast in concrete as a support, the shrinking of the concrete during curing may result in discontinuous support of the coil. In heater operation this would allow for microslapping of the coil against the concrete and consequent generation of even higher noise levels. To reduce the effect of the vibrating coil, coil supporting rubber or elastic layers have been interposed between the concrete casing and the coil to absorb the vibrational and expansion forces of the coil. However, a combination of a concrete casting and an elastic layer to support a coil completely fails in reducing the vibration of the coil itself and, thereby, permits the continued existance of the problems resulting from repeated coil flexing and consequent sound generation.
The present invention contemplates a new and improved apparatus which overcomes all of the above-referred to problems and others to provide a new induction heater for heating conductive workpieces which is simple in design, economical to manufacture, readily adaptable to a plurality of uses with workpieces having a variety of dimensional characteristics, is rugged and reliable in its operation, and which provides an improved induction heater in its reduction of coil vibration and sound generation.